This invention relates to power amplifiers and, in particular, to an improved power amplifying device capable of assuming selected circuit configurations.
In general, a typical power amplifier may consist of a series circuit comprised of a voltage amplifying stage for increasing the level of a signal applied thereto from a signal source and an impedance converting stage for changing the impedance presented to the amplified signal. In some instances, the impedance converting stage preferably has a high input impedance and relatively low output impedance. A typical example of such impedance converting stage is the emitter-follower amplifier. Since the gain of conventional voltage amplifying stage as well as the gain of a conventional impedance converting stage are susceptible to variations, the output signal level and output impedance of the power amplifier might correspondingly vary. Accordingly, if two power amplifiers are connected in parallel relation, the possible mismatch in gains and output impedance of the amplifiers might result in undesirable loading effects. In many applications, the output impedance of a power amplifier is selected to be relatively low. Thus, if two such power amplifiers are connected in parallel relation, the low output impedance of one such amplifier may present a load to the other amplifier thereby creating the possibility of an overload condition which could result in deleterious operation of the parallel configuration and quite possibly, serious damage to the overloaded power amplifier.
In other applications, the amplified signal produced by one conventional power amplifier might be supplied as an input signal to the other conventional power amplifier. The two power amplifiers operate upon the signals applied thereto so as to produce a pair output amplified signals admitting of opposite phases. If a load is coupled across the respective output terminals of the two power amplifiers, the resulting configuration is the so-called balanced transformer less connection. In the balanced transformerless connection, the load is connected in series to both power amplifiers. Accordingly, the load is provided with oppositely phased amplfied signals. The effective load applied to each power amplifier may be considered to be one-half of the total load. Hence, a balanced transformerless connection across a load is a useful circuit configuration for supplying an amplified signal to the load for those applications wherein the load is large. However, if the load coupled to the balanced transformerless connection is relatively small, the power amplifiers may then be subjected to undesirable loading effects which could result in damage to the amplifiers.
In those power amplifiers that include a voltage amplifyihng stage and a power amplifying or impedance converting stage, the voltage amplifying stage generally operates as a class A amplifier and the impedance converting stage that converts the impedance presented to the amplified signal from a high value to a low value generally operates as a Class B amplifier.